Routing table build algorithm for a routing table that uses a search key constructed from packet destination address and outbound interface

ABSTRACT

A method is described that involves, in view of a first default entry for a first subnet that is reachable through a first network service, where the first default entry has a numeric destination value, and in view of a second default entry for a second subnet that is reachable through a second network service, wherein the second default entry has the numeric destination value, sorting the first and second default entries by deciding that the first network service is ranked higher than the second network service. The method further involves configuring the first default entry within a routing table to have the numeric destination and not an interface component within the first default entry&#39;s search term, and, configuring the second default entry within the routing table to have the numeric destination and the second network service&#39;s interface component within the second default entry&#39;s search term.

CLAIM TO PRIORITY

This application claims priority from co-pending U.S. ProvisionalApplication Ser. No. 61/059,778, filed on Jun. 8, 2008, the disclosureof which is hereby incorporated by reference for all purposes.

BACKGROUND

FIG. 1 shows a prior art computing system 101 that is communicativelycoupled to a plurality of Local Area Networks (LANs) and Wide AreaNetworks (WANs). As observed in FIG. 1, each of LANs 103_1 through 103_4is a local area network to which the computing system 101 is directlyconnected. Examples of such local area networks include standard “Layer2” networks such as IEEE 802.3 based networks (e.g., Ethernet networks)and IEEE 802.11 based networks (e.g., Wireless/WLAN networks).

As depicted in FIG. 1, each of the LANs 103_1 through 103_4 is a subnetwithin a larger “Layer 3” (e.g., Internet Protocol (IP)) network. Notethat at least two separate Layer 3 networks 104_1, 104_2 are observed inFIG. 1 (e.g., a first IP network 104_1 that is separate and isolatedfrom a second IP network 104_2). The computing system is also coupled toWANs 104_3 and 104_4 (noting that each of Layer 3 networks 104_1, 104_2could also be WANs). The computing system 101 is directly connected toWAN 104_3 and remotely coupled to WAN 104_4.

The computing system 101 contains separate interfaces for communicationinto the subnets 103_1 through 103_4 and WANs 104_3 and 104_4. That is,interfaces 102_1 through 102_4 are interfaces for subnets 103_1 through103_4, respectively, and, interfaces 102_5 and 102_6 are interfaces forWANs 104_3 and 104_4, respectively. Here, each interface has: 1) an“end-point” address (e.g., an IP address) that corresponds to an addressof the computing system 101 on a particular network; and, 2)functionality for executing protocols that are specific to theparticular network.

Transmitting packets into a particular network is accomplished bysubmitting payload data to the corresponding interface. The interfaceand its associated functionality responds by encapsulating the payloaddata with appropriate protocol information to form a packet for itsrespective network. The existence of an interface therefore representsthe existence of a “network service” in the sense that the computingsystem maintains functionality for forming packets for transportationwithin a particular network.

FIG. 1 also shows a routing table 110 within the prior art computingsystem 101. The routing table identifies, based on an outbound packet'sdestination address, an appropriate network interface for that packet.In the case of subnets, the routing table also identifies the packet'sMAC layer destination address within the subnet. The followingdiscussion illustrates by way of example the manner in which routingtable entries are added and used according to the prior art approach.Specifically, FIGS. 2 a through 2 d pertain to the manner in whichentries for subnets 103_1 through 103_4 are added and used. FIGS. 2 ethrough 2 g pertain to the manner in which entries for WANs 104_3 and104_4 are used.

As observed in FIG. 2 a, two initial entries are observed 211 and 212for subnets 103_1 and 103_2—both of which are Ethernet LANs. As observedin FIG. 2 a, an entry for a LAN identifies: 1) the LAN's address as a“destination”; and, 2) the LAN's network interface within the system101. For instance, entry 211 identifies: 1) subnet address 192.168.1 asthe destination; and, 2) interface “en_1” which is the interface forsubnet 103_1.

As will be observed immediately below by way of example, once a subnetroute entry exists, the computing system is able to resolve for specificdestination addresses within the subnet. For the routing table state 210a observed in FIG. 2 a, when a need to send a packet to a particular endpoint destination or “host” on either of subnets 103_1 or 103_2 arises,the host's destination address is used as a look up parameter forperforming a “longest prefix match” search in the destination addresscolumn of the routing table 210 a. Thus, for example, if a need arisesto send a packet to an end-point within system 105_1 (located on subnet103_1) having destination address 192.168.1.10, a look-up is performedin the routing table 210 a using 192.168.1.10 as a look up parameter.The longest prefix match will hit on entry 211 (since 192.168.1 is adeeper match than the default entry).

The computing system 101 is able to recognize, however, that entry 211only identifies a subnet (192.168.1) and not a particular host on thatsubnet (192.168.1.10). As such, as observed in FIG. 2 b, the computingsystem adds another entry 213 in the routing table 210 b specificallyfor the host destination. Entry 213 is referred to as a “route” becauseit identifies a specific host or end-point destination. The addition ofnew route 213 is also consistent with the ARP protocol associated withIPv4 subnets in that it includes the additional detail of a specific IPdestination address (192.168.1.10) but, at least initially, onlyduplicates or “clones” the interface information (en_1) of the subnetnetwork service entry 211.

For new host routes within a subnet, such as route entry 213, thecomputing system 101 will attempt to further resolve the route 213 foradditional destination address information that is specific to thesubnet (e.g., a specific MAC layer address within the subnet).Specifically, the computing system 101 will launch an ARP packet intosubnet 103_1 to identify the MAC address for the system on subnet 103_1that includes IP address 192.168.1.10. Because system 105_1 has thisendpoint, the subnet 103_1 will return the Ethernet MAC address ofsystem 105_1.

Upon receipt of the MAC address for system 105_1 from subnet 103_1, thecomputing system 101 will enter the MAC address for system 105_1(00:aa:bb:cc:dd:ee) as the next node for entry 213. This state isobserved as state 210 c in FIG. 2 c. Thus, a subsequent look up ondestination 192.168.1.10 in the table will return the next node for thatdestination (i.e., MAC address (00:aa:bb:cc:dd:ee)) and the interfacefrom which a packet sent by computing system should be sent in order toreach that destination (i.e., interface “en_1” 102_1). Thus, the nextnode column in the routing table (where applicable) identifies, for aspecific entry in the routing table's destination column, theappropriate network node that the computing system 101 should send thepacket to.

If a need to send a packet to destination address 192.168.2.20 withinsystem 105_2 on subnet 103_2 arises, the same process repeats itselfresulting ultimately in new route entry 214 that specifies, as observedin FIG. 2 d, the MAC address of system 105_5 (00:11:22:33:44:55) as thenext node for that destination and the interface (“en_2” 102_2) fromwhich a packet sent by computing system should be sent in order to reachthat destination.

Here, both of entries 213, 214 identify an interface that transmits intoa subnet. (specifically, entry 213 identifies interface en_1 whichtransmits into subnet 103_1 and entry 214 identifies interface en_2which transmits into subnet 104_1) Properties of an interface thattransmits into a subnet include a capability to articulate any one ofmultiple destination addresses within the header information created bythe interface. As a consequence of this property, subnet representationswithin the routing table typically include multiple entries/destinationsfor a same subnet interface.

For instance, if additional end-point destinations 192.168.2.11,192.168.2.12 and 192.168.3.13 existed on subnet 103_2, three additionalentries would be observed in FIG. 2 d beneath entry 214—one entry foreach of these IP addresses that specifies its own unique MAC address.Here, over the course of operation, the subnet interface en_1 102_1 isexpected to be able to create header information that includes any oneof these MAC addresses as appropriate.

Interfaces for “non subnets” also exist. Examples include an interfacefor a Virtual Private Network (VPN) and an interface for aPoint-to-Point (PPP) link over a physical transport medium (such as PPPover a modem connection or PPP over Ethernet (PPPoE)). Becauseinterfaces such as these, from their own isolated perspective, do nottransmit into a subnet—they do not include the ability to identify arange of possible destinations within the header information theyconstruct. As a consequence, the structure of their correspondingrepresentations within the routing table differ from those of a subnet.

FIG. 1 supports some examples. Specifically, assume the WAN_1 interface102_5 is a PPP interface whose corresponding packets are transmittedover a direct connection to the Internet (WAN_1) 104_3, and, assume theWAN_2 interface 102_6 is a VPN interface configured to transmit packetsto a server 105_6 that provides access to a VPN WAN_2 104_4.

In the case of the PPP interface 102_5, which may be for a PPP overmodem connection or a PPP over Ethernet (PPPOE) connection into WAN_1104_3, the PPP interface 102_5 encapsulates IP packets destined fortransport within the WAN_1 with appropriate PPP headers and forwardsthem to an access node to WAN_1 (not shown) which strips off the PPPheaders and “dumps” the IP packets into WAN_1. The PPP headers createdby the interface 102_5 are not capable of uniquely identifying a rangeof possible destination addresses because all packets created by the PPPinterface 102_5 are sent directly to this access node.

The VPN interface 102_6 encapsulates packets for transportation withinWAN_2 104_4 with an IP packet whose destination address (10.11.12.13)specifies node 105_6. That is, WAN interface 102_6 is designed toconstruct a packet that will reach the access node 105_6 to WAN_2. Assuch, like the PPP interface 102_5, the header information created bythe VPN interface 102_6 is not capable of uniquely identifying a rangeof possible destination addresses because all packets created by the VPNinterface 102_6 are sent directly to the WAN_2 access node 105_6.

FIG. 2 e shows the presence of a routing table entries 215, 216 for theVPN interface 102_6 and corresponding WAN_1 104_4. Routing table entriesfor the PPP interface 102_5 and WAN_1 will be discussed in more detailfurther below. With respect to the VPN, two entries 215, 216 are createdin the routing table as observed in FIG. 2 e. A first entry 215 liststhe IP address of the VPN access node 105_6 (10.11.12.13) as adestination and the subnet gateway 106 (IP address=192.168.1.11) used toreach the access node as a next node that computing system 101 transmitsto. A second entry 216 lists the portion of a destination address thatidentifies WAN_2 (“17/8”) as a destination and identifies the WAN_2interface 102_6 as the appropriate interface.

Upon submission of these entries to the routing table, the followingoperations transpire. WAN_2 has address “17/8” so packets destined forWAN_2 will have an address that begins with “17/8”. For instance, apacket destined for node 105_7 will be labeled 17/8.1.2.3. Submission ofthis destination address to the routing table results in a hit on entry216 because that is deepest match amongst the destination addresseslisted in the routing table. Entry 216 identifies the WAN_2 interface102_6. As such, an outbound packet having destination address 17/8.1.2.3is processed by interface 102_6 which, as discussed previously, includesencapsulation with an IP packet having a destination address thatidentifies access node 105_6 (10.11.12.13). At this point an IP packethaving destination address 10.11.12.13 has been created which causes asecond lookup into the routing table.

Destination address 10.11.12.13 will hit on entry 215 which returns theidentity of gateway 106 (IP address=192.168.1.11). A lookup on IPaddress 192.168.1.11 returns a hit on entry 211 which causes, inaccordance with the manner in which entries 213 and 214 were created,the addition of an entry 217 for gateway 106 (having MAC address00:aa:bb:cc:dd:ff) within subnet 103_1. Thus, a lookup on entry192.168.1.11 ultimately returns the MAC address of gateway 106 and theidentity of the en_1 interface 102_1. Ultimately therefore, the outboundIP packet created by the WAN_2 interface 102_6 having the IP address ofthe VPN access node 105_6 is encapsulated with Ethernet headers by theen_1 interface 102_1 that specify the MAC address of gateway 106. TheEthernet packet is then transmitted into subnet 103_1. Subsequentpackets whose destination address specify network “17/8” will cause anidentical chain of processes (except that entry 217 does not need to becreated because it now exists in the routing table). In this manner, adestination address is continually resolved until no further resolutionof destination addresses and interfaces remain.

FIGS. 2 a through 2 f also show the presence of a default entry 218. Inthe prior art approach, the default entry 218 was used as a “catch all”to route packets whose destination addresses did not hit on a morespecific entry in the routing table into a particular network. Asobserved, the default entry 218 also identifies the gateway 106 forsubnet 103_1. Thus, as presently observed in FIG. 2 f, a packet whosedestination address fails to more specifically match to another routingtable entry will be forwarded into network 104_1 through gateway 106. Asan example, consider a packet whose IP destination address is 1.2.3.4.Submission of this destination address to the routing table results in adeepest match hit on the default entry 218 as compared to the otherentries observed in the routing table (in the prior art approach, thedefault entry was set to 0.0.0.0).

As a result of the hit on the default entry 218, the packet isencapsulated with Ethernet headers by the en_1 interface that specifythe MAC address of gateway 106 as the destination. Also, an entry 219for destination address 1.2.3.4 is entered into the routing tablelisting the gateway 106 of subnet 103_1 as the next node. Thus, asubsequent submission for destination 1.2.3.4 will hit on entry 219which returns the IP address of gateway 106 (192.168.1.11) which causesanother lookup in the routing table which hits on entry 217. The resultof this hit is the transmission of the packet from en_1 into subnet103_1 encapsulated with the MAC address of gateway 106.

A problem with the prior art approach, however, was that only onedefault entry was present in the routing table at any given timeresulting in, among other problems, the inability of the computingsystem 101 to transmit packets into all of the networks it was coupledto. For instance, PPP interface 102_5 is only useable if WAN_1 isconfigured to be the default. Here, recall that the VPN has a specificnumeric identifier “17/8” while WAN_1 has no such specific numericidentifier. In the present example being discussed, WAN_1 is theInternet which is a collection of different networks having differentnumeric identifiers but no single numeric reference identifies theInternet particularly. Thus, packets destined for the Internetessentially have random destination addresses (like 1.2.3.4) which—atleast initially—will only hit on the default entry. Thus, in the priorart approach, in order to send packets into network WAN_1 104_3, thedefault entry had to be changed from the state observed in FIG. 2 f tothe state observed in FIG. 2 g.

Here, FIG. 2 g shows the default entry 220 after it has been changed toWAN_1 (e.g., to configure Internet service for computing system 101). Abackground process also detected the change and removed entry 219(because that was presumed valid for a different network). Here,submission of an IP packet destined for the Internet 104_3 having anessentially random destination address (e.g., IP address=5.6.7.8) willhit on the default entry 220 which returns the identity of the PPPinterface WAN_1 102_5. The PPP interface appends PPP headers to the IPpacket and transmits the PPP packet to the access node for WAN_1 (notshown) which strips off the PPP headers and dumps the packet into theInternet/WAN_1. An entry 221 is created in the routing table fordestination address 5.6.7.8 that identifies the WAN_1 interface.

A problem with the prior art approach is that network destinations ondifferent networks which required the use of the default entry to bereached could not be concurrently reached. For instance, if network104_1 was isolated from the Internet, and another packet to the 1.2.3.4destination address on network 104_1 was submitted to the routing tableobserved in FIG. 2 f, the packet would improperly be sent into theInternet 104_3 rather than network 104_1 because the default entry 220no longer points to network 104_1.

Another problem is asymmetric input/output flows. For instance,computing system 101 can receive multiple input streams from network104_1 via its connection to that network through multiple interfacesen_1 through en_3. Because of the single default entry (e.g., gateway106), however, input flows arriving to interfaces en_1 through en_3 frombeyond their respective subnets 103_1 through 103_3 could only beresponded to through the same subnet via the default entry. So, forinstance, interface en_1 would transmit to all hosts beyond subnets103_1 through 103_3 even though interfaces en_, en_2 and en_3 werereceiving packets from these hosts (here, recall that each interface hasits own IP address on network 104_1). Interface en_1 would therefore berequired to process more than a pro rata share of the traffic beingreceived by system 101 resulting in a potential bottleneck.

Another problem was the inability to handle identical destinationaddresses on different networks. According to the manner in whichentries were added and listed in the routing table, a same destinationaddress would always be sent from the same interface.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 shows a networking implementation with a prior art routing table;

FIGS. 2 a through 2 g show pertinent aspects of the prior art routingtable;

FIG. 3 shows a networking implementation with an improved routing table;

FIG. 4 shows pertinent aspects of an embodiment of an improved routingtable;

FIG. 5 shows additional pertinent aspects of an embodiment of animproved routing table;

FIG. 6 shows an embodiment of an improved computing system having asocket containing an ifscope value and a cached route, and, a routelookup process that determines a scope value prior to performing a routelookup.

FIG. 7 shows a process for determining a scope value prior to performinga route lookup.

FIG. 8 shows a route lookup process that contemplates a search keyhaving a scope value.

FIG. 9 shows an implementation of a routing table and a process foradding entries to the routing table.

FIG. 10 shows an embodiment of a computing system.

DETAILED DESCRIPTION

FIG. 3 shows an improved architecture that includes a routing table 310which provides next nodal hop information in response to an input key320 formulated from a destination address and an indication of anappropriate outbound interface. Thus, in comparison to the prior artrouting table of FIGS. 2 a through 2 g which only receives a destinationaddress as a look-up key, the improved routing table 310 uses both adestination address and the identity of a specific interface as alook-up key 320. Because the look-up key 320 of the improved approachincludes more information than the look-up key of the prior artapproach, as explained in more detail below, the computing system 301 ofFIG. 3 is able to implement a number of possible improvements over theprior art system such as, among other possibilities, multiple defaultentries which permits proper handling of multiple networks having varieddestination address spaces, symmetric input/output flows, and correcthandling of identical destination addresses within different networks.Each of these features are described in more detail further below.

In order to provide an input key 320 that includes both a destinationaddress and an appropriate outbound interface, an additional functionallayer 330 is depicted in FIG. 3 residing “above” the routing table 310that has the responsibility of determining the correct outboundinterface in view of certain conditions under which an outbound packetis being transmitted. When the correct outbound interface is determined,its identity is essentially presented to the routing table 310 alongwith the outbound packet's destination address. Functional layer 330 canbe implemented in any number of ways.

For instance, according to a first “purely centralized” approach,applications or services within computing system 301 are configured torequest functional layer 330 to determine the correct outbound interfacefor an outgoing packet in view of the current set of conditions.According to a second “purely distributed” approach, applications orservices within computing system 301 are written to determine on theirown accord which outbound interface is proper for the outgoing packetsthey submit or handle. A continuum of architectures that invoke amixture of centralized and distributed approaches exist between thesetwo extremes.

Note the following features in FIG. 3: 1) hosts 305_1 and 305_2 areconnected to the same network 304_1 that interfaces 302_1 through 302_3are connected to but are reachable only through default entries; 2)subnets 303_1 through 303_3 are part of a different network 304_1 thanthe network 304_2 to which interface 302_4 is connected to; 3) hosts305_3 and 305_4, being respectively attached to different networks 304_1and 304_2, happen to have identical destination addresses. Thesefeatures are pertinent to some potential advantages of the improvedapproach that are described immediately below. A discussion of variousprotocols/methodologies for building the contents of the routing tableare discussed thereafter. FIG. 4 shows a routing table embodiment 410for the exemplary embodiment of FIG. 3.

Symmetric Input/Output Traffic Flows

The potential of the improved approach to engage in symmetriccommunication exchanges to hosts residing beyond a local subnet becomesevident through an analysis of communication exchanges between system301 and hosts 305_1 and 305_2. As mentioned just above hosts 305_1 and305_2 are connected to the same network 304_1 that interfaces 302_1through 302_3 are connected to but neither resides on a subnet to whichcomputing system 301 is directly coupled. As such, both of hosts 305_1and 305_2 must be reached through a default entry.

Assume that host 305_1 sends packets to the IP address associated withinterface 302_1, and, host 305_2 sends packets to the IP addressassociated with interface 302_2. Assuming the computing system 301 isable to entertain and continue respective communication sessions withhosts 305_1 and 305_2, note that the prior art approach by way of itssingle default entry will impose that all outgoing packets sent fromsystem 301 to hosts 305_1 and 305_2 must be sent over the same interfaceand subnet gateway that the single default entry identifies (e.g.,gateway 306 on subnet 303_1).

By contrast, functional layer 330 of the improved design may beconfigured to “track” which interface a communication session request orpacket arrived on and render a determination that outgoing packets forsuch communication sessions are sent over the same interface that theircorresponding request/packet arrived at (i.e., interface 302_1 forpackets to be sent to host 305_1 and interface 302_2 for packets to besent to host 305_2). As such, when computing system 301 preparesoutgoing packets for both hosts 305_1 and 305_2, the functional layer330 will determine that interface 302_1 is preferred for the packet tobe sent to host 305_1 and interface 302_2 is preferred for the packet tobe sent to host 305_2.

As such, a first search key will be built for the packet to be sent tohost 305_1 that is constructed from the destination address of host305_1 and the identity of interface 302_1. Likewise, a second search keywill be built for the packet to be sent to host 305_2 that isconstructed from the destination address of host 305_2 and the identityof interface 302_2. The first search key will hit on entry 411 (whichhas the destination address of host 305_1 and the identity of interface302_1 in the search column) of routing table 410 of FIG. 4 and thesecond search key will hit on entry 412 (which has the destinationaddress of host 305_2 and the identity of interface 302_2 in the searchcolumn) of routing table 410. Entry 411 identifies the MAC address ofthe gateway router 306 for subnet 303_1 as the next hop and entry 412identifies the MAC address of the gateway router 407 for subnet 303_2 asthe next hop. Thus the packet for host 305_1 will be sent to gatewayrouter 306 and the packet for host 305_2 will be sent to gateway router307.

It is pertinent to point out the manner in which entries 411 and 412were initially created. For entry 411, the initial outbound packet forhost 305_1 created an initial search key that included the destinationaddress of host 305_1 (1.2.3.4) and the identity of interface en_1. Thiscaused a deepest match hit on the default entry 401 for the en_1interface. As such, similar to the prior art process, new entry 411 wascreated that initially listed the IP address of the subnet 303_1 gatewayas the next node (192.168.1.11). Through the ARP process, the IP addressof gateway 306 was replaced in the next node column of entry 411 withthe Ethernet MAC address of the gateway 406. This is the form of entry411 as presented in FIG. 4.

Entry 412 was created in a similar fashion, noting however, that theinitial hit for a search key constructed from destination 5.6.7.8 anden_2 was the default entry 402 for subnet 303_2. This resulted in thegateway 307 for subnet 303_2 being identified as the next node for entry412. Thus, because both of subnets 303_1 and 303_2 have respectivedefault entries within the routing table, essentially both of interfacesen_1 303_1 and en_2 303_2 can support the transmission of essentiallyrandom destination address spaces.

Note that the routing table 410 of FIG. 4 is able to preserve thearrival/departure symmetry if a second set of packets from hosts 305_1and 305_2 arrive at reversed interfaces with respect to the previousexample (i.e., host 305_1 sends packets to the IP address of interface302_2 and host 305_2 sends packets to the IP address of interface302_1). In this case, using the same interface tracking algorithm,functional layer 330 will identify interface 302_2 as the appropriateinterface to send a packet to host 305_1 from and will also identifyinterface 302_1 as the appropriate interface to send a packet to host305_2 from. The corresponding search keys will hit on entry 413 for thepacket to be sent to host 305_1 and entry 414 for the packet to be sentto host 302_2. As such, the packet for host 305_1 will be sent intosubnet 303_2 to gateway router 307 and the packet for host 305_2 will besent into subnet 303_1 to gateway router 306. Entries 413 and 414 wouldbe created in like fashion as entries 411 and 412.

Other examples demonstrate the versatility of the present routingapproach. Consider (a perhaps unlikely) scenario where two applicationsresiding on computing system 301 are respectively bound to interfacesen_1 (302_1) and en_2 (302_2) and the applications send packets to oneanother over network 104 (e.g., the first application causes a packet tobe sent from en_1 to en_2 and the second application causes a packet tobe sent from en_2 to en_1). The sending of the initial packets wouldcause the addition of entries 403, 404 to the routing table (i.e., entry403 is for a packet being sent from en_1 to en_2 and entry 404 is for apacket being sent from en_2 to en_1).

As a further indication of the versatility of the present routingapproach, if two different interfaces are coupled to the same subnet,input/output traffic from/to the subnet can still be made symmetricalthrough the interfaces consistent with the principles described above.That is, if two interfaces have the same IP subnet address, the trackingalgorithm would identify the correct interface (i.e., the interface thatthe packet being responded to was received on) which, in turn, wouldtrigger the correct deepest match hit in the routing table. That is, asearch key of the form [[subnet_1.destination], interface_1] would hiton an entry of the form [subnet_1.x, interface_1] even though therouting table contained additional entries of the form [subnet_1.x,interface_2].

Identical Destination Addresses on Different Networks

Recall that the prior art approach, having only a single default entrywas not capable of correctly sending packets for different, isolatednetworks having identical destination addresses. The improved approachof FIG. 3 does not suffer from this drawback. As observed in FIG. 4, therouting table 410 includes multiple entries 415, 417 having an identicalIP destination address but for different networks 304_1, 304_2. Here,when multiple instances of the same destination address exist,functional layer 330 essentially selects the correct network through itsidentification of a particular interface. The routing table 510 of FIG.5 demonstrates this capability explicitly by showing fully resolveddestination addresses for hosts coupled to different networks.Specifically, as discussed above, entries 411 through 416 are fordestinations on network 304_1 while entry 417 is for destination 305_4on network 304_2. Note that entry 415 has the same destination addressas entry 417 (192.168.1.10).

Reachability

Recall that another problem in the prior was the inability to reach morethan one network whose destination address space could conceivably covera wide range of numeric values. That is, the default entry was used as amechanism to catch widely varied destination address values but, becausethe prior approach utilized only a single default entry, only a singlenetwork could be identified to handle a wide range of destinationvalues. Specifically, an example was presented where either an interfacefor network 104_1 or an interface 102_5 for network 104_3 could beassigned the default entry. Network 104_1 was a large IP network andnetwork 104_3 was the Internet. The interface 102_5 for WAN_1 was a PPPinterface that used a modem or Ethernet network as an underlyingphysical medium. Essentially, the problem manifested itself as aninability to concurrently send packets both on network 104_1 beyondsubnets 103_1 through 103_3 and on the Internet 104_3 because the singledefault entry could only be assigned to one of these networks.

FIG. 5 shows additional entries that may reside in the routing tablepresented in FIG. 4. Specifically, note the presence of a global or“unscoped” default entry 501 that does not identify a particularinterface and that returns the interface 302_5 for WAN_1 as theappropriate interface. This configuration essentially indicates that anydestination intended for the Internet, or for whom an appropriatenetwork/interface could not be identified, has a corresponding searchkey that does not add interface information. Such a search key will hiton entry 501 causing the corresponding packet to be sent into theInternet 304_3. Concurrently, however, outbound packets can still reachany destination on network 304_1 at least by way of default entries 401and 402 of FIG. 4. In this manner, a wide range of numeric destinationaddresses can be concurrently reached on more than one network.

Entries 502 and 503 correspond to entries added for a VPN network 304_4(WAN_2) that corresponds to the VPN WAN_2 104_2 discussed in thebackground. Normally, a VPN informs the system 301 of routes/addressesto be used within the VPN network 304_4. According to one embodiment,the computing system 301 links these routes with an interface throughwhich packets destined for the VPN will be sent. As such, a search keywill created that includes a numeric identifier of the VPN (“17/8” whichexists within the route itself) and the interface. The search key willhit on entry 503. Alternatively, if the computing system recognizes thatthe numeric identifier of the VPN is unique amongst the destinationaddress values it manages (i.e., no other networks contemplate adestination address that begins with “17/8”) entry 503 can remain“unscoped” (i.e., not include an interface identifier), and, routes aresubmitted to the routing table without any appended interfaceinformation. In this case entry 503 does not include interfaceinformation.

Either way, a hit on entry 503 returns the identity of the WAN_2interface 302_6. The WAN_2 interface is embedded with both the identityof the VPN access node 305_6 (IP address=10.11.12.13) and the identityof the interface from which packets destined for the VPN will betransmitted from (en_1 302_1). Thus, the WAN_2 interface encapsulatesthe packet for the VPN with an IP packet whose destination addressspecifies 10.11.12.13 and submits a search key to the routing table thatincludes this IP address and the identity of the en_1 interface. Thisresults in a hit on entry 502 which returns the IP address of the subnetgateway (192.168.1.11) and corresponding interface (en_1) of to thesubnet from where the packet will be transmitted. In this particularexample, the return of the next node information in entry 502 will beused to perform a lookup in the routing table that will hit on entry420. This causes the outbound packet to be encapsulated with the MACaddress of gateway 306 and transmitted from interface en1.

Determining the Interface Component of the Search Key

FIG. 6 depicts pertinent features concerning a process by which routeentries may be “looked up” from a routing table (such as the routingtable of FIG. 4). FIG. 6 shows a computing system 601 having aninterface 650 (I/F_1) coupled to a network 660. Communication between apair of computing systems coupled to one another through a network istypically accomplished through an exchange of packets over a connectionor session that is established by the two systems. A common approach isfor each system to internally establish a “socket” which represents thesession/connection (e.g., a software construct and/or program code).Internally on one of the systems, packets for transport over theconnection are delivered to the socket by an application engaged insubstantive communication to the other system. The socket callsappropriate networking services such as transport layer services andnetworking layer services.

FIG. 6 depicts an exemplary socket 620 within computing system 601 thatis used to implement a connection/session through interface 650.Notably, interface 650 may have one or more associated network addresses650 (e.g., IP addresses) where each such network address is a recognizedsource/destination end point on network 660. In operation, initially, anoutbound packet is presented to socket 620. The socket 620 calls atransport layer service (e.g., Transport Communication Protocol (TCP))and recognizes the building of transport layer header information on theoutbound packet. Next, the socket (or perhaps the transport layerservice) calls the networking layer for networking layer services.

The network layer service identifies the correct next hop address of thepacket and wraps the packet with, for instance, appropriate Layer 3(e.g., Internet Protocol (IP) and/or Layer 2 (e.g., Ethernet)destination headers. As discussed at length above, the present approachcontemplates the use of a routing table search key constructed from thepacket's destination and an outbound interface identifier. The routingtable (an example of which is observed in FIG. 4), accepts thisspecialized search key construct and returns next hop destinationinformation for the outbound packet as well as the outbound interface ofsystem 601 from which the packet should be transmitted.

According to one approach, meta data may be associated with (e.g.,included within) the socket 620 (or other representation of thecommunication session/connection) that specifies the interface to beidentified in the search key. For instance, FIG. 6 shows the presence ofan “ifscope” value 640 and a cached route 630 within the body of thesocket. The ifscope value is an identifier of an interface. The cachedroute is the result of a previous routing table lookup (i.e., next hopdestination and outbound interface identification) that has been storedin the socket 630. In practice, one, both or none of these items 630,640 may exist in the socket 620 when an outbound packet is presented tothe socket for outbound transmission. As will be explained in moredetail further below with respect to FIG. 7, the ifscope value 640 orthe outbound interface portion of the cached route 630 may be used toset the interface component of the search key (hereinafter referred toas the search key's or lookup's “SCOPE” value).

FIG. 6 nevertheless shows the process flow at a high level. According tothe process flow of FIG. 6, when the networking layer is called 663, theappropriate SCOPE value for the routing table search key is determined664. As will be observed with respect to FIG. 7, depending on thecircumstances, the SCOPE value may: 1) be a null value (i.e., nointerface is specified); 2) specify a particular interface (e.g., bybeing based on the ifscope 640 or interface component of the cachedroute 630); 3) specify the primary interface. Once the SCOPE value isdetermined 664, a search key is constructed from the outbound packet'sdestination address and the SCOPE value and the lookup into the routingtable is performed 665. The packet is then transmitted from thecomputing system 601 from the interface returned by the lookup table.

FIG. 7 shows an approach for determining the SCOPE value. According tothe flow diagram of FIG. 7, the meta data in which the ifscope andcached route are kept may also store a flag that, when set, indicatesthat a cached route is “valid”. That is, the cached route's interfacecomponent is safe to use for establishing the SCOPE value. Thus,according to the process of FIG. 7, if the flag is set 701, the cachedroute's interface component is used to set the SCOPE value 702. If theflag is not set, the next inquiry 703 is whether or not the socketcontains an ifscope value 640 (if it does not, the ifscope value is saidto be “NULL”).

An ifscope value is a convenient way to specify a preferred outboundinterface for a communication session or connection. For instance, ifthe communication session or connection is quasi-permanent (e.g., aquasi-permanent “pipe” is setup between the computing system and itscommunication partner), an ifscope value that identifies the outboundinterface through which the communication emanates may be written intothe socket or corresponding meta data. As another example, the outboundpackets emitted by an application may be “bound” to a particularinterface by configuring the application to submit packets to aparticular socket and writing an ifscope value within the socket thatidentifies the interface. As another example, an initial received packetto initiate a communication session/connection with the computing systemmay be processed through a particular socket. The interface that thepacket was received on is then identified in the socket (e.g., with aspecific ifscope value). Thereafter, any outgoing packets for thecommunication session/connection are directed to the socket therebybinding any such outgoing packets to the appropriate interface.

From FIG. 7, if the ifscope value is not null (i.e., an ifscope valueexists), the interface identified by the ifscope value is searched foran IP address that matches the source IP address of the outgoing packet.Here, recall that one or more IP addresses for a particular network canbe associated with a specific interface such that packets being sentto/from one of these IP addresses on the network will flow through theinterface. Thus, process 704 essentially attempts to confirm that theinterface identified by the ifscope value has an associated IP addressthat the outgoing packet is supposed to be sent from.

If there is no match, the packet is discarded and any cached routewithin the socket is discarded 706. If there is a match, the SCOPE valuefor the lookup search key is set to a value that corresponds to theinterface identified by the ifscope value 705. Next, if the socket doesnot contain a cached route (r=NULL) 707, the process is finished 708.Because the socket does not contain a cached route, the result of theimmediately following route table lookup will be stored in the socket asthe cached route. As such, conceivably, the next time an outbound packetis presented to the socket, the answer to inquiry 707 will be “no”(i.e., a cached route now exists). The cached route is then checked foran IP address that matches the source address of the outgoing packet709. That is, the validity of the routing table output is verified. Ifthe cached route's interface contains an IP address that matches thesource IP address of the outgoing packet, the flag is set 710 indicatingthe cached route is valid (as such, the next time a packet for thissession/connection is presented the SCOPE value determination processwill terminate at step 702 so as to avoid the time consuming searchingprocesses 704, 709).

If there is no match, the cached route is discarded 711 and the processis finished 713. Note that processes 709, 711 will also expunge anearlier, now stale cached route in the socket is used for a “new” ordifferent connection. That is, if the socket is used for a newconnection and updated with a new ifscope value but an earlier—nowinappropriate—cached route is not expunged from the socket, the firsttime a packet for the new connection is presented to the socket theanswer to inquiry 707 will be “no” and processes 709 and 711 willdiscard the stale cached route. The result from the routing table lookupwill then be cached in the socket such that the next packet for theconnection will trigger process 707 and 709 with the hope that the flaggets set 710.

Returning to process 703, note that it is possible that a socket simplymay not be configured with an ifscope value. If the socket does notcontain an ifscope value 703 or a cached route 714 the SCOPE value isset to null and the process is complete 715. In this case, as will beexplained in more detail below, the interface component of the routingtable lookup search key is set to a NULL value. If ifscope is null but acached route exists, an inquiry is made that checks whether there was arouting table change 716 since the cached route was embedded in thesocket. If the routing table has not been changed, SCOPE is set to avalue that reflects the interface component of the cached route 717.Then, the interface identified by the cached route is checked for an IPaddress that matches the source IP address of the outgoing packet 719.

If there is a match, the flag is set 721 and the process is complete722. If a match does not result, the different interfaces within thecomputing system are searched continuously looking for an interfacehaving the packet's source IP address 723. If no such interface isfound, the packet is dropped and the cached route is discarded 724. Ifsuch an interface is found, the SCOPE value is changed to a value thatidentifies the interface found through process 723. At this point theprocess is complete, however, the result of the subsequent route tablelookup is cached in the socket.

Returning back to process 716, if there was a routing table change sincethe cached route was embedded in the socket, the SCOPE is set to a valuethat identifies the primary network (if one exists). The primaryinterface is then checked to see if it has an associated IP address thatmatches the source IP address of the outgoing packet. If it does, theflag is set 727 and the process is complete 728. If it does not, process723 and the subsequent processes as described above are performed.

Routing Table Lookup

Returning back to FIG. 6, after a SCOPE value is determined 644 a routelookup is performed 665 with a search key constructed from thedestination address and the SCOPE value. FIG. 8 shows an embodiment of aprocess for performing the route lookup. According to the process ofFIG. 8, initially, an “unscoped” route lookup is performed 901. Asalluded to above with respect to FIG. 5, and as will be discussed againfurther below with respect to FIG. 9, the routing table may include both“scoped” and “unscoped” entries. Scoped entries are entries whosecorresponding search key includes a true interface component. Bycontrast, unscoped entries are entries whose corresponding search keyincludes a NULL interface component. Search keys having a non NULLinterface value will therefore match on scoped entries and search keyshaving a NULL interface value will match on unscoped entries. Accordingto an implementation, two different values “r” and “r0” are both set tothe result of the unscoped route lookup.

If the interface component of the search key is NULL and there is noprimary network, the unscoped lookup result (r or r0) is returned as thefinal result 802. If the interface component of the search key is set toNULL and there is a primary network the SCOPE value is set to a valuethat corresponds to the primary network's interface 803. Here, the“primary” network's interface corresponds to the interface of the globaldefault route. So, in the example of FIGS. 4 and 5, the primary networkinterface corresponds to the WAN_1 interface (see entry 501 of FIG. 5).In the case where the interface component of the search key is not setto NULL, an inquiry is made to see if the unscoped lookup 801 returned aNULL value or not. If the unscoped route lookup 804 did not produce aNULL result (i.e., r, r0 contain a destination address and outboundinterface) and the SCOPE value is NULL 805, the SCOPE value is reset toa value that corresponds to the interface component of r, r0 806. Bycontrast, if the unscoped route lookup 804 did not produce a NULL result(i.e., r, r0 contain a destination address and outbound interface) andthe SCOPE value is not NULL 805, r0 value is cleared leaving only r 807.

Regardless, the existence of a SCOPE value that is not NULL causes ascoped looked to be performed 808. The result is entered into r, thus,if r0 still exists, there exists both an r value (scoped route result)and an r0 value (unscoped route result). If the scoped lookup result ris NULL 809, the value of r is set equal to r0 (thus r =r0) 811. If thescoped result r is not NULL 809, an inquiry is made into the specificityof r vs. the specificity of r0 810. For instance, a Layer 2 result(e.g., the MAC address on an Ethernet subnet) is deemed to be morespecific than a Layer 3 result (e.g., an IP address). If r is not morespecific than r0, r is set equal to r0. Next, or r is more specific thanr0, an inquiry is made into the status of r. If r is not null, theprocess is complete 817 and r is returned as the route lookup result.

If r is null, the destination component of the search key is set to thedefault value (0.0.0.0) and another lookup is performed 813 to produce anew value for r. If r is a NULL 814, the process is complete and routelookup returns a NULL as a final result 817. If the result is not aNULL, the interface component of the result is compared against theSCOPE value. 815 If they match, the process is complete and r isreturned as the lookup result. If they do not match, r is set to NULL,the process is complete and a NULL value is returned as the lookupresult.

Routing Table Build

FIG. 4 disclosed an embodiment of a routing table that served thepurpose of easily describing a routing table that can receive a searchkey composed of a destination and an interface identifier. FIG. 9 showsanother embodiment of such a routing table. The routing table depiction900 of FIG. 9 is composed of “rearranged” entries that appear in FIG. 4and FIG. 5.

Referring to FIG. 9, the “top” 901 of the routing table includes the“global” default entry 501 of FIG. 5. Recall that the global defaultentry can be used as a “catch-all” for specific destinations that do nothave an entry in the routing table but that nevertheless can be reached.For instance, the global default entry 901 may identify an outboundinterface through which the Internet is reached (because the Internetsupports a vast number of destination addresses that the routing tablecannot list the entire set of). In operation, the routing table returnsthe next hop and outbound interface for the entry whose search columncomponent has the “longest prefix match” with the search key. Thus, inan embodiment, the default entry 901 has a 0.0.0.0 search columndestination component because essentially random destination addresses(such as those associated with the Internet) will exhibit a closestmatch with 0.0.0.0 as opposed to the specific addresses listed elsewherein the table.

A subnet will introduce two entries to the table, a “default” entry forthe subnet and a “subnet entry” for the subnet. As observed in FIG. 9,for subnet 192.168.1, entry (401) corresponds to the default entry andentry 920 corresponds to the subnet entry. Here, default entry (401) ofFIG. 9 essentially corresponds to default entry 401 of FIG. 4. Likewise,the subnet entry 920 for subnet 192.168.1 in FIG. 9 is represented inFIG. 4 just above the default entry. An analogous set of default andsubnet entries (402), 930 are also observed in FIG. 9 for subnet192.168.2.

Comparing FIGS. 4 and 9, however, note that the subnet default entries(401), (402)—like the global default entry 901—are represented with adestination value of 0.0.0.0. The routing table physically lists entriesin numerical order. That is, search column entries with lower numericvalues will reside above search column entries with higher numericvalues. Because the default entries 901, (401), (402) have the lowestnumeric destination values in the table (0.0.0.0) they reside at the topof the table. The global default entry 901 is “unscoped” (i.e., does nothave an associated interface value in its search column) whereas thesubnet default entries (401) and (402) are “scoped”. That is, entry(401) has an identifier of interface en_1 in its search column and entry(402) has an identifier of interface en_2 in its search column.

According to an embodiment, a number of “sort policies” are applied tothe entries in the table so that their order in the table is properlyimplemented. A first sort policy, as mentioned just above, is to sortentries based on the numerical value within the search column. A secondsort policy is that amongst a collection of entries having identicaldestination, one of these entries is left unscoped while the remainingentries are scoped. Both of these policies can be observed in thedefault entries 901, (401), (402) of FIG. 9.

With respect to the first policy, entry 901 does not have an interfacecomponent in its search column and therefore is numerically less thanentries (401) and (402). Entry (401) is numerically less than entry(402) because interface component “en_1” is presumed to be numericallyless than interface component “en_2”. The same principle is observedwith respect to the remaining table entries observed in FIG. 9.

With respect to the second sort policy, each of entries has adestination value of 0.0.0.0. Therefore, according to the policy, onemust be chosen as the unscoped entry. In an embodiment, the manner ofchoosing the unscoped entry is as follows. Firstly, an “automatic”sorting policy is applied based on the type of network service.According to one approach, the automatic sorting policy ranks networkservices more prone to be Internet connections or otherwise connected tonetworks having a wide range of potential destination addresses areranked higher than network services that are less prone to be Internetconnections or supportive of a large destination address range. Forexample, the following automatic sorting policy is emblematic of such anapproach: 1) serial-based PPP (greatest propensity to be an Internetconnection); 2) Ethernet; 3) Firewire; 4) wireless (least propensity tobe an Internet connection).

Above and beyond the automatic sorting policy, however, specific rankingcriteria may be explicitly applied to any route. For instance, accordingto one embodiment, any route may be labeled as one of: 1) “sort first”;2)“sort last”; 3) “never unscoped”. Reviewing the default entries 901,(401), (402), it is clear that entry 901 has been chosen to be theunscoped entry over and above entries (401) and (402). Any of thefollowing scenarios could have established this sort order:

-   -   1) the network service of entry 901 (through interface WAN_1)        was deemed by the automatic sorting policy to have a greater        propensity to be an Internet connection than the network        services of entries (401) and (402) (Ethernet)—and—the explicit        rank labels (first, last, never unscoped), if any, did not        overrule the automatic sorting (i.e., neither of entries (401),        (402) were ranked “first” and entry 901 was not ranked “sort        last” or “never unscoped”);    -   2) the network service of entry 901 was ranked “sort first”;    -   3) the network service of entry (401) and/or (402) was ranked        “sort last” or “never unscoped”; etc.

When a search key having a SCOPE of NULL is presented to the routingtable, only entry 901 (being the only unscoped entry observed in thetable) is within the set of entries that are searched for a longestprefix match. The remaining entries observed in the routing table 900 ofFIG. 9 are not searched when SCOPE is NULL because the remaining entrieshave an interface component within their respective search column entry.That, is an unscoped search key will only return an uscoped searchresult. In practice multiple unscoped entries are possible, however,because there may be additional instances of identical destinationcomponent but different interface component (e.g., entry pair 411 and413 of FIG. 4, entry pair 412 and 414 of FIG. 4, etc). The same sortingcriteria described above would be applied to such entries (e.g., one ofentries 411 and 413 would be unscoped).

In operation, the routing table of FIG. 9 is utilized much the same asthe table observed in FIG. 4. That is, for example, an unscoped,essentially random Internet destination will have a deepest match onentry 901 and therefore be processed by WAN_1. As another example, adestination of 1.2.3.4 and a scope of en_1 will have a deepest match onentry (401) which returns the gateway (192.168.1.11) for subnet192.168.1 (303_1), which causes a second scoped lookup that will have adeepest match on entry (420). A “new” destination for subnet 192.168.1(303_1) (e.g., 192.168.1.9 with scope en_1) will have a deepest match onentry 920 which will cause an address inquiry into subnet 192.168.1(303_1) for the MAC layer address on that subnet and a new entry to beinserted (just above entry (415)) in the table. Note that if there wereadditional interfaces each configured to be on subnet 192.168.1 and192.168.2 respectively then there would be both an unscoped and scopedentry for 192.168.1 and 192.168.2.

FIG. 9 also shows a methodology by which the routing table is populated.According to the methodology of FIG. 9, when a new entry is to be addedto the table, the “initial” location is identified 950 by itsdestination 901. For example, if another default entry of destination0.0.0.0 were to be added to the table, the region of entries 901, (401),(402) would be initially identified as the correct realm for insertionof the entry. The sorting criteria would then be applied 960. Forinstance, if the new entry was labeled “rank first” it would displaceentry 901 at the top of the table.

As a final, additional comment. Recall from the discussion of FIG. 7that an interface might be searched through to discover what sourceaddress it supports (e.g., box 704 of FIG. 4). In an embodiment, thesource IP address of a route is embedded in the route entry in thetable. In this manner searches can be performed on a particularinterface that returns all of that interface's supported source IPaddresses.

Closing Comments

FIG. 10 shows one example of a typical computing system (or “computersystem”) which may be used with the present invention. Note that whileFIG. 10 illustrates various components of a computer system, it is notintended to represent any particular architecture or manner ofinterconnecting the components as such details are not germane to thepresent invention. It will also be appreciated that personal digitalassistants (PDAs), cellular telephones, handheld computers, mediaplayers (e.g. an iPod), entertainment systems, devices which combineaspects or functions of these devices (e.g. a media player combined witha PDA and a cellular telephone in one device), an embedded processingdevice within another device, network computers, a consumer electronicdevice, and other data processing systems which have fewer components orperhaps more components may also be used with or to implement one ormore embodiments of the present invention. The computer system of FIG.10 may, for example, be a Macintosh computer from Apple Inc. The systemmay be used when programming or when compiling or when executing thesoftware described.

As shown in FIG. 10, the computer system 45, which is a form of a dataprocessing system, includes a bus 51 which is coupled to a processingsystem 47 and a volatile memory 49 and a non-volatile memory 50. Theprocessing system 47 may be a microprocessor from Intel which is coupledto an optional cache 48. The bus 51 interconnects these variouscomponents together and also interconnects these components to a displaycontroller and display device 52 and to peripheral devices such asinput/output (I/O) devices 53 which may be mice, keyboards, modems,network interfaces, printers and other devices which are well known inthe art. Typically, the input/output devices 53 are coupled to thesystem through input/output controllers. The volatile memory 49 istypically implemented as dynamic RAM (DRAM) which requires powercontinually in order to refresh or maintain the data in the memory. Thenonvolatile memory 50 is typically a magnetic hard drive, a flashsemiconductor memory, or a magnetic optical drive or an optical drive ora DVD RAM or other types of memory systems which maintain data (e.g.large amounts of data) even after power is removed from the system.Typically, the nonvolatile memory 50 will also be a random access memoryalthough this is not required. While FIG. 10 shows that the nonvolatilememory 50 is a local device coupled directly to the rest of thecomponents in the data processing system, it will be appreciated thatthe present invention may utilize a non-volatile memory which is remotefrom the system, such as a network storage device which is coupled tothe data processing system through a network interface such as a modemor Ethernet interface. The bus 51 may include one or more busesconnected to each other through various bridges, controllers and/oradapters as is well known in the art.

It will be apparent from this description that aspects of the presentinvention may be embodied, at least in part, in software. That is, thetechniques may be carried out in a computer system or other dataprocessing system in response to its processor, such as amicroprocessor, executing sequences of instructions contained in amachine readable storage medium such as a memory (e.g. memory 49 and/ormemory 50). In various embodiments, hardwired circuitry may be used incombination with software instructions to implement the presentinvention. Thus, the techniques are not limited to any specificcombination of hardware circuitry and software nor to any particularsource for the instructions executed by the data processing system. Inaddition, throughout this description, various functions and operationsare described as being performed by or caused by software code tosimplify description. However, those skilled in the art will recognizewhat is meant by such expressions is that the functions result fromexecution of the code by a processor, such as the processing system 47.

It is worthwhile to point out that although the above routing tablescenarios were described in relation to a single computing system, anumber of such computing systems configured with the routing techniquesdescribed herein could be coupled to one or more networks, and, witheach such system using the routing techniques discussed herein, engagein communication with one another.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

1. A method, comprising: in view of a first default entry for a firstsubnet that is reachable through a first network service, said firstdefault entry having a numeric destination value, and in view of asecond default entry for a second subnet that is reachable through asecond network service, said second default entry having said numericdestination value, sorting said first and second default entries by:deciding that said first network service is ranked higher than saidsecond network service; and, configuring said first default entry withina routing table to have said numeric destination and not an interfacecomponent within said first default entry's search term; and,configuring said second default entry within said routing table to havesaid numeric destination and said second network service's interfacecomponent within said second default entry's search term.
 2. The methodof claim 1 wherein said first network service is used to send to trafficinto the Internet.
 3. The method of claim 2 wherein said first networkservice is a PPP network service.
 4. The method of claim 2 wherein saidsecond network service is any of: An Ethernet service; A Firewireservice; A wireless service.
 5. The method of claim 1 wherein said firstnetwork service was labeled to be ranked first.
 6. The method of claim 1wherein said second network service was labeled to be ranked last. 7.The method of claim 1 wherein said second network service was labeled tonot be scoped.
 8. The method of claim 1 wherein said method furthercomprises adding a first subnet entry for said first subnet into saidrouting table and adding a second subnet entry for said second subnetinto said routing table.
 9. The method of claim 8 wherein said firstsubnet entry's search term includes said first subnet's address and saidsecond subnet entry's search includes said second subnet's address. 10.The method of claim 9 wherein said first subnet entry's search term alsoincludes said first network service's interface and said second subnetentry's search term also includes said second network service'sinterface.
 11. A machine readable storage medium containing program codewhich when processed by a processing unit causes a method to beperformed, said method comprising: in view of a first default entry fora first subnet that is reachable through a first network service, saidfirst default entry having a numeric destination value, and in view of asecond default entry for a second subnet that is reachable through asecond network service, said second default entry having said numericdestination value, sorting said first and second default entries by:deciding that said first network service is ranked higher than saidsecond network service; and, configuring said first default entry withina routing table to have said numeric destination and not an interfacecomponent within said first default entry's search term; and,configuring said second default entry within said routing table to havesaid numeric destination and said second network service's interfacecomponent within said second default entry's search term.
 12. Themachine readable storage medium of claim 11 wherein said first networkservice is used to send to traffic into the Internet.
 13. The machinereadable storage medium of claim 12 wherein said first network serviceis a PPP network service.
 14. The machine readable storage medium ofclaim 12 wherein said second network service is any of: An Ethernetservice; A Firewire service; A wireless service.
 15. The machinereadable storage medium of claim 11 wherein said first network servicewas labeled to be ranked first.
 16. The machine readable storage mediumof claim 11 wherein said second network service was labeled to be rankedlast.
 17. The machine readable storage medium of claim 11 wherein saidsecond network service was labeled to not be scoped.
 18. The machinereadable storage medium of claim 11 wherein said method furthercomprises adding a first subnet entry for said first subnet into saidrouting table and adding a second subnet entry for said second subnetinto said routing table.
 19. The method of claim 18 wherein said firstsubnet entry's search term includes said first subnet's address and saidsecond subnet entry's search includes said second subnet's address. 20.The machine readable storage medium of claim 19 wherein said firstsubnet entry's search term also includes said first network service'sinterface and said second subnet entry's search term also includes saidsecond network service's interface.
 21. An apparatus, comprising: inview of a first default entry for a first subnet that is reachablethrough a first network service, said first default entry having anumeric destination value, and in view of a second default entry for asecond subnet that is reachable through a second network service, saidsecond default entry having said numeric destination value, means forsorting said first and second default entries comprising: means fordeciding that said first network service is ranked higher than saidsecond network service; and, means for configuring said first defaultentry within a routing table to have said numeric destination and not aninterface component within said first default entry's search term; and,means for configuring said second default entry within said routingtable to have said numeric destination and said second network service'sinterface component within said second default entry's search term.